J . Am. Chem. SOC.1991,113, 8698-8704
8698
Photochemical Core Manipulation in High-Nuclearity Os-Hg Clusters L. H. Cade,? B. F. G. Johnson,? J. Lewis,*,?M. McPartlin,*vt T. Kotch,s and A. J. Lees*,! Contribution from the University Chemical Laboratory, Lensfield Road, Cambridge CB2 I E W, U.K.,School of Chemistry, The Polytechnic of North London, Holloway Road, London N7 8DB, U.K.,and Department of Chemistry, State University of New York at Binghamton, P.O.Box 6000, Binghamton, New York 13902-6000. Received March 21, 1991 Abstract: The cluster dianion [ O S , , H ~ , C , ( C ~ ) ~ , (2) ] ~ -has been synthesized in over 90% yield by reacting [OS,~C(CO)~,]~-
with 1.6 equiv of Hg(02CCF3)2,and the molecular structure of its [PPN]+ salt ([PPN]+ = bis(triphenylphosphine)iminium(l+)) was determined by X-ray crystallography. The cluster contains a central Hg, triangle fusing two tricapped-octahedral Os9 fragments together: triclinic, Pi, a = 19.022 (4) A, b = 15.737 (3) A, c = 15.204 (3) A, a = 115.66 (2)O, p = 68.32 (2)O, y = 115.45 (3)O, V = 3612.2 A3, Z = 1, R = 0.0859. 2 undergoes partial demercuration on irradiation with visible light to generate the corresponding dimercury cluster dianion [ O S , , H ~ , C ~ ( C O ) ~(3), ~ ] which ~ - thermally reincorporates metallic mercury quantitatively, thus making 2 a photochromic system. The structure of 3 contains a dimercury fragment sandwiched between the two Os9subclusters in a uniquely compact way. Both dianions 2 and 3 can be reversibly oxidized to their corresponding monoanions that are related to each other by a similar reversible photochemical process. Quantum yields for the photolytic Hg extrusion for both systems were obtained from spectral sequences after successive irradiation with the 458-, 488-, and 514-nm lines of an Ar ion laser. They lie between 0.004 and 0.006 for the dianionic and between 0.003 and 0.005 for the monoanionic cluster, reflecting the efficiency of nonreactive deactivation pathways in these systems. Irradiation with UV light at 337 nm (N, laser) favors the competing C O dissociation leading to a drop in the quantum efficiency of the demercuration by an order of magnitude.
There is a n ever growing number of structurally characterized high-nuclearity clusters containing more than ten metal atoms,’ while their chemical behavior, especially the manipulation of their metal core, remains almost unexplored. An underlying problem inhibiting the systematic investigation of their reactivity is the delicate balance between their metal-metal and metal-ligand bond structures which, if disrupted, may lead t o more or less complete disintegration of the system. A way out of this dilemma is the buildup of cluster structures in which t h e metal core is locally labilized, i.e. activated only with respect to certain metal-metal bond cleavages, thereby allowing specific structural modifications under appropriate reaction conditions. Mixed-metal clusters containing late transition or main group metal atoms with significantly different electronic properties from those of the principal core metal meet these requirements. T h e resultant polar metal-metal bonds are potentially intrinsic sites of chemical or even photochemical reactivity. W e have recently synthesized and studied a number of Os-Hg clusters derived from reactions of the cluster dianion [OsloC(C0),,l2- (1) with a range of different mercury electrophiles.2 Through careful selection of the solution properties of the H g reagents the generation of a variety of novel structural types, containing up t o 21 metal atoms, was achieved. On going, for example, from the highly dissociated salt H g ( 0 3 S C F 3 ) , to the corresponding trifluoroacetate and by modification of the stoichiometry a complete change of the reaction pathway was effected. While the former generated the Hg-linked cluster [(OsloC(CO),,J2Hg]’ in good yields (eq 1) the latter yielded the cluster [ O S , ~ H ~ , C , ( C O ) ~ ,(2) ] ~ -very selectively (eq 2). T h e full details
of the structure of the monomercury-linked dianion and of other mercury-containing products, specifically obtained by appropriately tuning the reagents and reaction conditions, a r e being published separately.2b T h e octadecaosmium-trimercury cluster 2 contains
a triangular Hg, subunit, sandwiched between two tricappedoctahedral O S ~ C ( C O fragments )~~ derived from 1 by removal of a capping Os(CO), vertex (Figure T h e system has clearcut interfaces between the different metallic domains and therefore, in the context of the strategy outlined above, appeared to be the ideal model system for reactivity studies specifically involving the metal core. T h e observation of the light sensitivity of 2 in solution was the starting point for a systematic study of its photolysis and the related ~ h e m i s t r y . ~Photochemically induced reactions affecting either the metal core or ligand shell of high-nuclearity clusters are almost unexplored. Research in this field has so far focussed on trinuclear and, to a lesser extent, tetranuclear systems with ~ ~e r~e specific emphasis on ligand activation and s u b s t i t ~ t i o n . H (1) Representative examples: (a) Chini, P. J . Orgonome!. Chem. 1980, 200, 37. (b) Martinengo, S.; Sironi, A,; Chini, P. J . Am. Chem. SOC.1978, 100,7076. (c) Ciani, G.; Magni, A.; Sironi, A.; Martinengo, S. J . Chem. Soc., Chem. Commun. 1981, 1280. (d) Martinengo, S.; Ciani, G.; Sironi, A. J . Am. Chem. SOC.1980, 102, 7564. (e) Jackson, P. F.; Johnson, B. F. G.; Lewis, J.; Nelson, W. J. H.; McPartlin, M. J . Chem. SOC.,Dalton Trans. 1982, 2099. (f) Ceriotti, A.; Demartin, F.; Longoni, G.; Manassero, M.; Marchioni, M.; Piva, G.; Sansoni, M. Angew. Chem., Int. Ed. Engl. 1985,24,697. (9) Fenske, D.; Ohmer, J.; Hachgenei, J. Angew. Chem., Int. Ed. Engl. 1985,24,993. (h) Ceriotti, A.; Fait, A.; Longoni, G.; Piro, G.; Demartin, F.; Manassero, M.; Masciocchi, N.; Sansoni, M. J . Am. Chem. SOC.1986. 108,8091. (i) Mednikov, E. G.; Eremenkov, N. K.; Slovokhotov, Y . L.; Struchkov, Y.T. J . Chem. SOC.,Chem. Commun. 1987,218. 0’) Teo, B. K.; Hong, M. C.; Zhang, H.; Huang, D. B. Angew. Chem., Inf.Ed. Engl. 1987,26,897. (k) Charalambous,
E.; Gade, L. H.; Johnson, B. F. G.; Lewis, J.; McPartlin, M.; Powell, H. R. J . Chem. SOC.,Chem. Commun. 1990,688. (I) Amoroso, A. J.; Gade, L. H.; Johnson, B. F. G.; Lewis, J.; Raithby, P. R.; Wong, W. T. Angew. Chem., Inf. Ed. Engl. 1991, 30, 107. (2) (a) Gade, L. H.; Johnson, B. F. G.; Lewis, J.; McPartlin, M.; Powell, H. R. J . Chem. SOC.,Chem. Commun. 1990, 110. (b) Gade, L. H.; Johnson, B. F. G.; Lewis, J.; McPartlin, M.; Morris, J.; Powell, H. R. J . Chem. SOC., Dalron Trans. Submitted for publication. (3) The Ru analogue has also recently been prepared: Bailey, P. J.; Johnson, B. F. G.; Lewis, J.; McPartlin, M.; Powell, H. R. J . Chem. SOC., Chem. Commun. 1989, 1513. There is, however, at present no indication of a similar photochemical behavior on irradiation with visible light: Bailey, P.
J. Personal communication. (4) Charalambous, E.; Gade, L. H.; Johnson, B. F. G.; Kotch, T.; Lees, A. J.; Lewis, J.; McPartlin, M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1137. (5) (a) Gallop, M. A,; Johnson, B. F. G.; Lewis, J.; McCamley, A,; Perutz, R. N. J . Chem. SOC.,Chem. Commun. 1988, 1071. (b) Malito, J.; Markiewicz,
s.;Poe, A. J. Inorg. Chem. 1982, 21, 4335.
(c) Po& A. J.; Sekhar,
C. V. J . A m . Chem. SOC.1986, 108, 3673. (d) Bengtsen, J. G.; Wrighton, ‘University Chemical Laboratory. *The Polytechic of North London. rState University of New York at Binghamton.
0002-7863/9 1/ 15 13-8698$02.50/0
M. S. J . A m . Chem. SOC.1987, 109, 4518. (e) Bengtsen, J. G.; Wrighton, M . S. J . A m . Chem. SOC.1987, 109, 4530. (f) Desrosiers, M. F.; Wink, D. A.; Trautman, R.; Friedmann, A. E.; Ford, P. C. J . A m . Chem. SOC.1986, 108, 1917.
0 199 1 American Chemical Society
Photochemical Core Manipulation in Os-Hg Clusters
J . Am. Chem. Soc.. Vol. 113, No. 23, 1991 8699
[ O S ~ ~ H ~ ~ C ~precipitated ( C O ) ~ ~which ] was isolated by removal of the liquor with a cannula. The remaining solution was evaporated to dryness, and the residue extracted twice with methanol and recrystallized from dichloromethane-diethyl ether to yield a further 38 mg of 2. Total yield: 403 mg (93%). Anal. Calcd for CII,H60P4N20s,sHg3:C, 22.09; H, 0.95; N, 0.44. Found: C, 22.01; H, 0.94; N, 0.46. IR(CH2C12)Y 2072 (m), 2057 (s), 2005 (s). I3C NMR (62.8 MHz, 50% "C-enriched samI092 ple, recorded in CD2C12at 295 K) 6 = 185.7 (6 CO), 184.9 (6 CO), 177.7 072 (12 CO, 2J(i99Hg13C)= 56 Hz), 175.6 (18 CO), 397.4 (interstitial carbide). The most abundant isotopomers for the cluster anions in the negative ion FAB mass spectrum (NBA matrix) were the following: dianion m/z 2615 (stimulated 2615), monoanion m / z 5228 (simulated , ,A = 540 nm. The combined 5230). Absorption spectrum (CH2CI2) methanol extracts were evaporated to dryness to yield [PPN] [Os(C0),(02CCF3),] as a beige powder. IR(CH2C12) u(C0) 2127 (s), 2038 (br, vs). I3C NMR (100.6 MHz, CD2C12)6 = 167.5 (CO), 162.0 (0COCFI, 'J("F"C) = 37.2 Hz), 114.6 (-CF,, 'J(I9Fl3C) = 289.2 Hz. Negative ion FAB mass spectrum: 615 [MI-, 587 [M - COJ-, 559 [M - 2CO]-, 474 [M - C O - CF,CO,]-, 446 [M - 2CO - 2CFIC02]-. [PPN]2[0s18Hg2C2(C0)42] (3). A stirred solution of [PPNJ2[?sIsHg,C,(CO),,] (50 mg, 0.0082 mmol) in CH2CI2was irradiated with a 250-W tungsten lamp with the reaction vessel being externally cooled with water. After ca. 10 min, precipitation of metallic mercury was observed, and the reaction was completed after 40-50 min as judged from the IR spectra recorded during the reaction. During this time the color of the solution changed from deep purple to a much less intense burgundy red. After separation of the mercury by filtration the solvent volume was reduced to 15 mL and the product crystallized by slow vapor diffusion with diethyl ether. Black crystals, yield 45.5 mg (92%). Anal. Calcd for Cl16H~P4N20slsHg2: C, 22.82; H, 0.98; N, 0.49. Found: C, 23.01; H, 1.17; N, 0.51. IR(CH2C12)u(C0) 2064 (s), 2057 (vs), 2007 (s). I3C NMR (100.6 MHz, 50% "C-enriched sample, recorded in CD2CI2at 295 K) 6 = 185.8 (6 CO), 185.7 (6 CO), 179.7 (12 CO, 2J(t99Hg13C) not resolved), 176.0 (I8 CO), 41 1. I (interstitial carbide). The most abundant F j p r e 1. The molecular structure of the cluster dianion in 2. For clarity isotopomers for the cluster anions in the negative ion FAB mass spectrum all atoms are drawn as spheres of arbitrary radius; the oxygen and carbon (NBA matrix) were the following: dianion m/z 2514 (simulated 2514.5), atoms of each C O group have the same number. monoanion m / z 5028 (simulated 5029). Absorption spectrum (CH2C12) , , ,A = 550 nm. we present the syntheses of [OS,~H~,C~(CO)~~]~(2) and its Reinsertion of Metallic Mercury into 3. [PPNI2.3 (20 mg), prepared photolysis product [ O S , ~ H ~ ~ C ~ ((3) C Oand ) ~a~ detailed ]~as described above, was dissolved in IO mL of CH2CI2and one small drop photochemical investigation of the reaction leading to 3. Full (ca. IO mg) of mercury metal was added to the system. Rapid stirring in the dark led to complete regeneration of 2 within a period of up to 4 details of the X-ray crystal structure of the [PPN]+ salt of 2 are h as judged by IR spectroscopy. A similar experiment was carried out reported. A preliminary report of the strucutre analysis of the with "C-enriched species, and the quantitative regeneration of 2 was [PPN]+salt of the dimercury cluster 3, which suffers from disconfirmed by "C NMR spectroscopy. If the experiment is carried out order, has already been p~blished;~ so far it has proved impossible in a more polar solvent than CH2CI2such as acetone, the reinsertion may to obtain better crystals of 3. remain incomplete (typically 80-90% regeneration of 2 in acetone). [PPN10slsHg,C2(C0)421(6). [PPN12[0slsHg3C2(CO)421(20 mg, Experimental Section 0.0032 mmol) was dissolved in 20 mL of CH2CI,. To this solution solid Preparation of Compounds. All manipulations were performed under Ag(02CCF,) (0.8 mg, 0.0036 mmol) was added. Within ca. 30 s the an inert gas atmosphere of dried argon in standard (Schlenk) glassware. color of the solution changed from purple to brown-red and IR spec[PPN ]2[Os,oC(CO)24]([PPN]+ = bis(triphenylphosphine)iminium( 1 +)) troscopy indicated a complete conversion to the monoanion. The comwas prepared as reported in the literature.lC All other chemicals were pound could be quantitatively precipitated as a black powder on addition obtained commercially and used without further purification. Diof pentane. Anal. Calcd for C~OH30P4N20s1sHg3: C, 16.66; H, 0.53; chloromethane used in the syntheses was dried over CaH2, distilled, and N, 0.25. Found: C, 16.29; H, 0.70; N, 0.34. IR(CH2CI2) u(C0) 2085 saturated with argon or N2 before use. The %enriched samples used (m), 2072 (s), 2022 (s). Most abundant isotopomer of the molecular ion for the I3C NMR studies reported in this paper were synthesized from in the negative ion FAB mass spectrum: m / z 5227 (simulated 5230). I3C-enriched OS,(CO),~.The enrichment of the osmium carbonyl was Absorption spectrum (CHZC12)?- = 526 nm. Exposure of the solution achieved as follows: Finely powdered OS,(CO),~(1.5 g) was suspended to moist air led to the immediate and complete regeneration of the in 60-70 mL of dry toluene in a 250" carius tube with an attached starting material. For the photolysis sequences the material was not Young tap. The solvent was degassed by three successive freezeisolated but generated in situ. [PPN].6 may alternatively be generated pump-thaw cycles and then pressurised to 1 atm with "CO (97-99%). by oxidation of [PPN],.Z with 1 equiv of [(4-BrC6H4),N][SbC16] in The suspension was heated at 130 'C (at which temperature all the THF. OS,(CO),~dissolved) in the closed tube for 3 days. The solution was then [PPN][Os18Hg2C2(C0)42] (7). (a) The solution of [PPN] [OslsHg3cooled down to room temperature and the solvent evaporated under C,(CO),,] prepared as described above was irradiated with a 250-W vacuum. The enriched O S , ( C O ) ~was ~ obtained as a yellow microcrystungsten lamp while being cooled by water from the outside of the retalline solid. The enrichment in l3CO achieved in a single cycle was ca. action vessel. Within ca. 40 min the color changed from brown-red to 30% as determined by mass spectroscopy. A second cycle yielded an green and IR spectroscopy indicated complete conversion to the di-50% enriched product. mercury cluster. On exposure to moist air the dianion 3 is generated [ P P N ~ O S , ~ H ~ ~ C ~( 2( )C. Note: O ) ~ ~The ] synthesis of this compound immediately. The solid compound could be obtained quantitatively by was carried out under rigorous exclusion of light! 1 (500 mg, 0.1365 precipitation with pentane. Anal. Calcd for CsoH30P4N20slsHg~: C, mmol) was dissolved in CH2C12(40 mL) and solid Hg(02CCFJZ(94mg, 17.25; H, 0.55; N, 0.26. Found: C, 17.03; H, 0.71; N,0.34. IR(CH2CI2) 0.2184 mmol = 1.6 equiv) added to the solution. After being stirred for u(C0) 2081 (s), 2073 (vs), 2022 (s). Most abundant isotopomer of the 1 h the solvent volume was reduced to ca. 25 mL and the reaction mixture molecular ion in the negative ion FAB mass spectrum: m/z 5027 (simwas left standing at room temperature for seven days and then stored at = 570 nm. (b) ulated 5029). Absorption spectrum (CH,Cl2) , , ,A -25 OC overnight. Within this period 368 mg (85%) of pure [PPN]2[ P P N ] , [ O S ~ ~ H ~ ~ C , ( C(20 O )mg, ~ ~ ]0.0032 mmol) was dissolved in 20 mL of CH2C12. To this solution solid Ag(02CCF3) (0.8 mg, 0.0036 mmol) was added. The color of the solution changed from burgundy red to green within ca. 1 min, generating the monoanion with the same ( 6 ) Bantel, H.; Hansert, B.; Powell, A. K.: Tasi, M.; Vahrenkamp, H. analytical and spectroscopic properties as above. The solid compound Angew. Chem., Inr. Ed. Engl. 1989, 28, 1059. 071
Q
Gade et al.
8700 J . Am, Chem. SOC.,Vol. 113, No. 23, 1991 Table I. Crvstal Data and Experimental Details for 2
formula wt, amu cell a, A
b, A
c,
A
v,A I
a,deg
& deg ,r, deg
6387.65 19.022 (4) 15.737 (3) 15.205 (3) 3609.2 I 15.66 (2) 68.32 (2) 115.45 (3)
2 1 temp, K 298 2.939 dcalc. g/cml space group P i (NO.2) crystal shape irregular crystal vol. mm3 0.018 radiation Mo K a ( A 0.7 1069 A) p.0 cm-' 183.6 scan speed, deg/s 0.50 scan method e12e scan width (0). deg 0.80 std reflcnsb three, scanned every 5 h 3-25 B range, deg data collected fh,fk,l p factor 0.04 no. of unique data 4546 (F: 2, 3u(F:)' no. of parameters" 387 0.086 R o n F, R , on F, 0.086 a Empirical absorption corrections (see: Walker, N.; Stuart, D. Acta Crystallogr. Sect. A 1983, 39, 158) were applied in two stages, the first after location of only the metal atoms and their refinement with isotropic thermal parameters. This facilitated location of all non-hydrogen atoms, and after their inclusion, with the H-atoms at idealized positions, the absorption correction procedure was repeated with the original data. *Initial studies showed the crystals to decompose rapidly in the X-ray beam apparently due to loss of solvent: in final data collection the crystal was coated with epoxyresin and there was no significant change in the standards. cData collection and processing were by standard methods which have been described previously (see: Jackson, Lewis, J.: Nelson, W. J. H.; McPartlin, M. J . P. F.; Johnson, B. F. G.; Chem. SOC.,Dalton Trans. 1982, 2099). "Structure solution and refinement was carried out with S H E L X ~ (g. ~ Sheldrick, Cambridge University, 1976).
The phenyl rings of the cations were assigned idealized hexagonal geometry (C-C = 1.395 A and C-H = 0.95 A) and refined as rigid groups with fixed thermal parameters of 0.10 A2assigned to the hydrogen atoms. I n the final stages of full-matrix refinement anisotropic thermal parameters were assigned to the metal and phosphorus atoms and individual weights of l/a2(F,) were assigned to each reflection (see Table 1). Final positional and isotropic thermal parameters are given in Table 11, anisotropic thermal parameters are in Table 111, a list of IOF, vs IOF, is given in Table IV, selected bond lengths are listed in Table V , and interbond angles are given in Table V I (all in supplementary material). Photolysis Experiments. Photolysis experiments at 448, 488, and 514 nm were carried out with a Lexel Corp. Model 95-4 4-W argon ion laser. Typical laser power was in the 25-50" range, and the incident light intensities were measured by means of an external Lexel Corp. 504 power meter. Irradiations at 337 nm were performed with a National Research Group, Inc. Model 0.5-5-150B pulsing nitrogen laser calibrated at 5 mJ/pulse with repetition rates of 30-60 pulses/s. In all photolysis experiments the concentrations of reactant and product were monitored throughout the reaction with a Hewlett-Packard 8450A UV-visible spectrophotometer which utilizes a rapid scanning microprocessor-controlled diode array detector. Typically, absorption spectra were obtained from solutions within 5 s of completing the irradiation. Absorption band maxima are considered accurate to f 2 nm in the visible region and & I nm in the ultraviolet region. Fourier-transform infrared (FTIR) spectra were also recorded during photolysis on a Nicolet 20 SXC FTIR spectrometer. In the photochemical experiments the sample solutions were prepared in CH2C12(HPLC grade) which had been deoxygenated by a freeze-pump-thaw method prior to use. The solutions were filtered through 0.22 fim Milipore films and transferred to a I-cm quartz cell via an air-tight syringe. During laser photolyses the samples were rapidly stirred to ensure that the solution exhibited homogeneous light absorptions. Photochemical quantum yields (acr)were determined by monitoring the disappearance of reactant 2 or 6 at their respective visible absorption maxima and by application of eq 3 which accounts for the changing degree of light absorption and for inner-filter effects. Here, [R] is the d[R]/df = -@crIo(l- l o - D ) f ~ b [ R ] / D
(3)
concentration of the reactant cluster at varying photolysis time t , Io is the intensity of the incident light per unit solution volume, f R and D a r e the molar extinction coefficients of the reactant cluster and the solution optical density at the photolysis wavelength, respectively, B is the cell path length, and a,, is the reaction quantum yield. Plots of In [(D,D,)/(Do - D,)] vs 1:,[(1 - 10-D)/D] dt, where D,, D,, and Do are the optical densities throughout the photolysis sequence at the reactant's visible absorption maximum, gave straight lines of slope -*&fRb. Quantum yield values were determined in triplicate and found to be within *8%. Other Physical Measurements. Infrared spectra were recorded on could be obtained quantitatively by precipitation with pentane. Perkin-Elmer Models 283B and 1710 FT-IR spectrometers. I3C NMR X-ray Diffraction Study of [PPN]2[0s,8Hg,C2(C0)42J'CH2C12. The spectra were recorded on Bruker AM400 and WM250 NMR spectrompreliminary examination of the crystal on a Philips PWI 100 four-circle eters and the negative ion fast atom bombardment mass spectra on a diffractometer indicated the triclinic crystal system, and this was conKRATOS MS 50 TC spectrometer (calibrant CsI). firmed by Delauney reduction. The unit cell parameters were obtained by least-squares refinement of the setting angles of 25 reflections. The Results and Discussion crystal data and experimental details are summarized in Table I . Synthesis and Structure of [PPN]2[0~18Hg3C2(C0)42]. The A Patterson synthesis was solved, assuming the centrosymmetric space reaction of [PPN]2[0s,oC(C0)24] (1) with 1.5-1.6 molar equiv group Pi,to give the fractional coordinates of six osmium atoms defining of Hg(02CCF3), leads to an almost complete conversion to an an octahedron. The positions of the remaining metal atoms were located Os-Hg carbonyl cluster with the stoichiometry [PPN],[OsI8in a subsequent observed-Fourier synthesis showing that each cluster dianion has the metal core of virtual D3 symmetry shown in Figure 1. Hg3C2(CO),,] (2) and the mononuclear complex [PPN] [Os(CThere was a 50:50 disorder of the central mercury triangle giving an O)3(02CCF3)3](4) (eq 2). An initial intermediate is the moimage of virtual D,, symmetry in the diffraction study with a "Star of noanion [ O S , ~ C ( C O ) ~ , ( H ~ O ~ C C (5), F ~which, ) ] - based on the David" arrangement of the six half-weight mercury atoms; this may be similarity of its IR spectrum to that of the X-ray crystalloattributed to a random distribution of the two possible cluster enantiomgraphically characterized species [Os,oC(CO),4(HgCF3)]-" and ers throughout the crystal. Close scrutiny of the Patterson synthesis did [ O S , , C ( C O ) , ~ ( H ~ B ~ is ) ] thought - , ~ ~ to have a similar structure not distinguish interatomic maxima due to vectors between the two with the mercury atom bridging three Os atoms of a capping orientations of the Hg, triangle; this is inconclusive as to the nature of tetrahedron. The formation of 5 is followed by what appears to the disorder due to the very large number of much higher Os-Os vector be a complicated cascade of reaction steps involving, among others, peaks. Attempts 10 proceed with the structure determination of the noncentrosymmctric space group PI were precluded by the dominance the nucleophilic attack of CF3CO; on the activated capping Os of the eighteen centrosymmetrically related osmium atoms. All nonatom, as suggested by the nature of the second reaction product hydrogen atoms were located in subsequent difference-Fourier syntheses, 4. I3C NMR spectroscopic studies of the reaction mixture showed and their parameters refined satisfactorily in the centrosymmetric space that while 2 is formed within minutes after the addition of the group. Two maxima, separated by about 3 A, of approximately half the mercury salt, conversions of over 90% are only obtained after height expected for chlorine atoms where located outside the bonding reaction times of several days. range of the dianion and the cations. These were included as chlorine The structure of the cluster dianion 2 is illustrated in Figure atoms of half occupancy, and a dichloromethane solvate molecule was 1 and principal bond distances are listed in Table V. In the crystal indicated by the location of a small maximum, assigned as a carbon atom of half occupancy at bonding distance from the two chlorine atoms. the cluster has virtual D3 symmetry. The core of 21 metal atoms
Photochemical Core Manipulation in Os- f f g Clusters Table V. Principal Bond Lengths (A) for 2 2.867 (3) 2.864 (4) OS(1 )-OS(3) OS( I )-OS(2) OS(1 )-OS(5) 2.854 (3) 2.844 (3) OS(I )-@('I) Os(l)-Os(9) 2.763 (3) 2.773 (3) Os(l)-Os(7) Os(Z)-Os(S) 2.857 (4) 3.065 (3) Os(2)-Os(3) 0 ~ ( 2 ) - 0 ~ ( 9 ) 2.837 (3) 0 ~ ( 2 ) - 0 ~ ( 6 ) 3.056 (3) 0~(3)-Os(4) 2.877 (3) Os(2)-Hg(3) 2.696 ( 5 ) 3.067 (3) Os(3)-Os(7) 2.833 (4) Os(3)-Os(6) Os(3)-Hg( 1 ) 0~(3)-Hg(3) 3.472 (6) 2.761 ( 5 ) 0 ~ ( 4 ) - 0 ~ ( 5 ) 2.844 (3) Os(4)-Os(6) 2.873 (3) 0 ~ ( 4 ) - 0 ~ ( 7 ) 2.756 (3) Os(4)-Os(8) 2.746 (3) 0 ~ ( 5 ) - 0 ~ ( 6 ) 2.863 (3) Os(5)-Os(8) 2.763 (3) 0 ~ ( 5 ) - 0 ~ ( 9 ) 2.755 (3) Os(6)-Os(8) 2.823 (4) Os(6)-Hg(2) Hg( I)-Hg(2) 2.931 (6) 2.678 (6) Hg(lkHg(3) Hg(2)-Hg(3) 2.931 (6) 2.920 (7) Os-C (carbonyl) range 1.53 (9)-2.06 (7) Os-C (carbide) range 1.95 (5)-2. I8 (5) C - 0 range 1.02 (9)-1.29 (7)
can be divided into three metallic domains: the two tricapped octahedral O S ~ C ( C O fragments, )~~ originating from the starting material 1, and the Hg, subcluster which fuses them together. Each Os,fragment of the cluster possesses a metal core geometry which until recently has eluded detection in many studies of binary osmium carbonyl systems and, as such, represented a missing link in the structural systematics of these compounds.' The mean twist angle between the central Hg3 unit and each of the two adjacent Os3 triangles of 30" results in these faces (Os(2)-0~(3)-0s(6) and Os(2')4~(3')-0~(6')) of the two Os9 fragments being staggered with respect to each other. In the crystal the two Osg fragments are exactly related by an inversion center, but the overall metal core does not contain this symmetry element due to the arrangement of the mercury atoms. The arrangement of the structural subunits generates a chiral axis through the centers of both Osg-cluster fragments with a possible left- or righ-handed twist of the central three-metal triangles. The (PPN]+ salt of the cluster crystallizes as a racemic mixture of the two enantiomeric forms randomly distributed throughout the crystal, resulting in a 5050 disorder of the Hg, units. The most remarkable structural feature of the cluster is the central Hg, triangle. The mean Hg-Hg distance of 2.927 (6)A indicates that the bonding between the mercury centers is rather weak, while their structural significance lies in the linking of the Os9subclusters. On the basis of the polyhedral skeletal electron pair theory, an electron count of 122 is required for each Os9 fragment.* Thus. in an extreme approach, a formal tetraanionic charge would be assigned to each nonaosmium moiety, resulting in the formal assignment of the Hg3 unit as a hexacation. This situation is similar to that in the "raft" cluster [{Os3(C0),,~,Hg3J~ where again all valence electrons of the central Hg, triangle are required for each Os3(CO)Il subunit to conform to the 18-electron rule; the mean H Hg distance of 3.100 (3) A is even longer than 2. In both these clusters the Hg-Hg distance that of 2.927(6) does not rule out a mere dIo-d1Ointeraction between the Hg(I1) centers.I0 Mercury compounds with the same structural arrangement of three Hg atoms, but lower formal charge, show significantly shorter bond lengths, as for instance in the recently described ( HgJ4+ clusters in Hg3(dppm),(S04), (d = 2.764-2.802 A)ll or the mineral terlinguaite, Hg4CI2O2(2.703 A).12 As can
I-in
(7) The cluster dianion [oSp(CO)24]2has recently been isolated in very small yields from pyrolysis mixtures and structurally characterized by X-ray crystallography: Amoroso, A. J.; Johnson. B. F. G.; Lewis, J.; Raithby. P. R.; Wong. W.-T. J . Chem. Soc.. Chem. Commun. 1991, 814. (8) (a) Wade, K. In Transition Metal Clusters; Johnson, B. F. G., Ed.; J. Wiley: New York. 1980. (b) Mingos. D. M. P. Acc. Chem. Res. 1984, 17. 31 1. (c) McPartlin. M. Polyhedron 1984, 3. 1279. (9) Fajardo. M.; Holden. D. H.; Johnson, B. F. G.; Lewis, J.; Raithby. P. R. J . Chem. Soc.. Chem. Commun. 1984, 24. (IO) Schmidbaur. H.; Oller, H. J.; Wilkinson. D. L.; Huber, B.; Miiller. G . Chem. Ber. 1989. 122. 31 and references cited therein. ( I 1 ) Hiimmerle. B.: Mirller, E. P.; Wilkinson. D. L.; Miiller, G.: Peringer. P. J . Chem. SOC..Chem. Commun. 1989. 1527. (12) Brodcrsen, K.; GBbel. G.; Liehr. G. 2. Anorg. AIIg. Chem. 1989.575. 145.
J . Am. Chem. SOC.,Vol. 113, No. 23, 1991 8701
Figure 2. Computed space-filling model of [Os,gHg3C2(CO)r2J2-.showing the exposed situation of the central trimercury unit.
3
1 I
Figure 3. 62.8-MHz I3C NMR spectrum o f the 500/0 ')C-enriched cluster 2 recorded in CD2CI2 a t 295 K.
be seen in the computed space filling model in Figure 2, the ligand-free central region of the cluster is remarkably exposed despite slight displacement of the twelve carbonyl ligands adjacent to the mercury unit toward the central triangle which results in a number of short C(carbonyl)--Hg contacts (range 2.559-2.857 A). The cluster is nevertheless relatively inert with respect to attack by bulky nucleophile^.^^ The symmetry and ligand distribution of the cluster is reflected in the I3C NMR signal pattern of the 50% I3C-enriched cluster (Figure 3). The four signals due to the carbonyl ligands have an intensity ratio of 1 :1 :2:3,corresponding to a 6:6:12:18 carbonyls. The most intense signal (6 = 175.6) is the resonance of the Os(CO), groups occupying the vertices. As in the case of all high-nuclearity Os and Ru clusters studied so far, there is a fast exchange of the CO's attached to the vertex atoms between the three geometrical positions ("Os(CO), rota tion") which renders them equivalent on the NMR time scale at ambient temperatures. The signal at 6 = 177.7 is due to the 12 C O S coordinated to the two Os3 triangles adjacent to the central trimercury unit. This assignment is supported by the set of IwHg satellites associated with the signal. The 2J(1wHg13C) coupling constant of 56 Hz is comparable to the 2J(199Hg13C) observed for [ R U ~ C ( C O ) ~ ~ (AuPPh,)(HgCI)] (51 Hz).I4 The two signals at 6 = 184.9 and ( 13) Another example of a cluster containing a sterically exposod mercury atom has recently been reported: Schoettel. G.; Vittal. J. J.; Puddephatt, R. J. J . Am. Chem. SOC.1990, 112,6400. (14) Cowie. A. G. Ph.D. Thesis, Cambridge University, 1984.
8702 J . Am. Chem. SOC.,Vol. 113, No. 23, 1991 n
0
U
V
Figure 4. The previously reported structure of the cluster dianion in 3, redrawn from ref 4, showing the relative "slipping" of the O s 9 subclusters.
185.7 are the resonances of the axial and equatorial carbonyl ligands on the Os(CO), fragments in the basal triangles. Synthesis of [PPN]2[0s18Hg2C2(CO)42] (3). In the course of our investigations into the reactivity of 2, it was found that when solutions of the cluster were exposed to light their color changed from deep purple to a much less intense burgundy red. Simultaneously, metallic mercury was generated and precipitated. The photochemical reactivity of compounds in which Hg is bound to a transition metal has been known for some time;Is however, systematic investigations of the reactions occurring are rare.16 The observation that, on exposure to light, 2 underwent a clean conversion to a single product, 3, prompted a detailed study of the nature of this process. The light-induced loss of a single Hg atom was confirmed by a negative ion FAB mass spectrum, while the similarity of the IR absorption band patterns of 2 and 3 in the CO-stretching region and the signal pattern in the I3C NMR spectra of both compounds suggested similar overall symmetries. The new cluster, 3, was thus formulated as [PPN]2[0s18Hg2C2(C0)421. The molecular structure of 3, established earlier from X-ray structure analysis of its slightly disordered [PPN]+ salt: is shown in Figure 4. The two nonaosmium subunits already present in 2 remain almost unchanged, but their open Os, triangles, while remaining parallel, are markedly "slipped" from the staggered arrangement observed in 2. Each of the two Hg atoms links an edge of one Os3 triangle with a corner of the other, generating a hitherto unknown structural environment for a dimercury fragment. The latter is embedded in the metal frame in a uniquely compact way in comparison to other examples of mixed-metal compounds containing Hg2 units.17 The reported Hg-Hg distance of 2.744 (5) A is considerably shorter than the mean Hg-Hg bond length of 2.927 (6) A in 2; although the magnitude of this dif( I 5 ) Burlitch, J. M. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A,, Abel, E. W., Eds.; Pergamon Press: New York, 1982; p 983. (16) Vogler, A.; Kunkeley, H. J . Organomel. Chem. 1988, 355, 1 . (17) (a) Albinati, A,; Moor, A,; Pregosin, P. S.; Venanzi, L. M. J . Am. Chem. Soc. 1982, 104, 7672. (b) Ghilardi, C . A,; Midollini, S.; Moneti, S. J . Chem. Soc.. Chem. Commun. 1981, 865.
Gade et ai. ference indicates a real shortening of the Hg-Hg distance, the observed Hg-Hg length in 3 must be treated with caution due to slight unresolved disorder of the Hg, units4 The photoinduced partial demercuration of 2, Le., the extrusion of a single mercury atom, may be viewed as an intramolecular redox reaction in which the remaining core of 20 metal atoms has undergone a formal two-electron oxidation. Thus, 3 is 'electron deficient" according to simple electron counting rules, and the slipped structural arrangement of the subclusters, which reduces the perpendicular separation of the open Os3 triangles by 0.3 8, with respect to 2, may be a consequence of this electronic situation. This view is supported by the fact that on reduction of 3 to the tetraanion [Os18Hg2C2(C0)42]4the metal core is straightened up again and the separation of the two centroids of the ~ S & ( C O ) ~ , fragments increased by ca. 0.3 .&I8 The I3C NMR spectrum, recorded at 295 K, with its four sharp carbonyl resonances corresponds to a higher symmetry (&) with respect to a molecular axis through the centers of both Os units than that displayed in the solid-state structure. We attribute this to a fast rotation of the two Os9 subclusters relative to each other on the NMR time scale in solution. This interesting observation of core fluxionality is also observed in other high-nuclearity Os-Hg clusters.19 As in the I3C N M R spectrum for 2 the signal of the interstitial carbide was observed, although significantly shifted to lower field (from 6 = 397.4 for 2 to 6 = 41 1.1 for 3). If, under exclusion of light, 3 is exposed to metallic mercury, thermal reinsertion of an Hg atom into the metal core occurs, regenerating 2 almost quantitatively within a period of ca. 6-8 h. This process takes place both with the in situ photolytically generated mercury and on addition of mercury to pure 3 isolated previously. The complete thermal reversibility of the photolysis makes 2 the first example of a photochromic high-nuclearity cluster. Synthesis of the Monoanions [ O S , ~ H ~ , C ~ ( C (6) O ) ~and ~~ [Os18Hg2C2(C0)42r (7). The dianion 2 may be oxidized by silver salts or other powerful oxidants such as [(4-BrC6H4)3N][SbC16] which to the corresponding monoanion [ O S ~ ~ H ~ ~ C ~ ((6) CO )~~]is stable in solution under an inert gas atmosphere, while on exposure to moist air 2 is regenerated immediately. The C O absorption band pattern in the infrared spectrum of 6 is similar to that of 2 but shifted by ca. 15 cm-I to higher wave numbers, a shift characteristic of a change by one charge unit in clusters of this size. Additionally, the difference in the cation:anion ratio on going from 2 to 6 was reflected in the microanalytical data obtained for both compounds. FAB mass spectra of 6 generated with silver salts gave no indication of a possible incorporation of silver into the metal core of the cluster. On exposure to light the brown-red solution of 6 in CH2C12turns deep green, a process that is reversed after removal of the light source. The green cluster 7 has the same IR spectroscopic characteristics as 3 but with the set of three sharp infrared bands shifted in the same was as those of 6 with respect to 2. If the green solution of 7 is exposed to moist air, the compound is immediately reduced to 3 with a concomitant change in color to burgundy red. On the other hand, oxidation of 3 with Ag+ salts regenerates 7 quantitatively. FAB mass spectra of 7 showed the same molecular ion peak as those of 3 and gave no indication of a possible incorporation of silver into the metal core of the cluster. The anion 7 may therefore be formulated as [Osl,Hg2C2(CO)42]-,a formulation consistent with the microanalytical data obtained for its [PPN]+ salt. Thus, the clusters 2, 3, 6, and 7 are part of the reversible redox and photochemical cycle shown in Scheme I. The Photolysis of [PPN]2[0S,8Hg3C2(CO)42] (2). The clean photolytic conversion of 2 to the demercurated product 3 can be conveniently monitored by either UV-vis or IR spectroscopy as shown in Figure 5. The most significant change in the UV-vis spectrum is the decrease in intensity and bathochromic shift by 10 nm of the absorption band at 540 nm (e = 4.38 X IO4 L mol-I (18) Gade, L. H.; Johnson, B. F. G.; Lewis, J.; McPartlin, M. J . Chem. Soc., Dalton Truns. Submitted for publication. (19) Gade, L. H.; Johnson, 8 . F. G.; Lewis, J . Unpublished results.
J . Am. Chem. SOC.,Vol. 113, No. 23, 1991 8103
Photochemical Core Manipulation in Os-Hg Clusters Scheme I. The Reversible Photochemical and Redox Chemical Cycle Linking 2, 3, 6, and 7"
hV
a
1
g
0
1
i
-
.
v)
n
a
Hg (metal)
04-
O
LA 400
-ip-iA--i'
500
700
800
Wavelength, nm
b
Ag+
600
(*I
Table VII. Quantum Yields for the Hg Extrusion of andO [) O Complexes ) , ~ ] - at 293 K [OS,~H~~C~(C ~ S~ ,]~~H- ~ ~ C ~ ( C O complex A,, nm I 03acr(l [Osi,HgK2(C0)al2- (2) 458 5.3 488 5.1
OReported values are in each case the mean of triplicate measurements.
cm-I). Absorption bands in this energy range are due to metal-metal transitions within the cluster core. The high nuclearity of the system investigated and, consequently, the high density of electronic states prohibits a detailed analysis of the spectral features. However, since the structural changes that go along with the photochemical reaction are primarily confined to the mercury domain and its interfaces with the rest of the core, we believe that the intense absorption band which is so drastically affected in the process is at least partially due to metal-centered charge-transfer transitions to Os-Hg or Hg-Hg antibonding cluster valence orbitals. An additional feature is what appears to be an increase of the absorbance over the whole spectral range which is particularly marked at longer wavelengths. This is probably a consequence of light scattering by a suspension of colloidal mercury formed during the photolysis. The retention of sharp isosbestic points indicates that the photodemercuration proceeds without interference by secondary photoprocesses. This is confirmed by the sequence of 1R spectra (Figure 5b) recorded after successive radiation intervals. The decrease in intensity of the u(C0) infrared band at 2072 cm-' and the growing in of the band at 2064 cm-l are convenient indicators for the conversion of 2 to 3. Quantum yields of the reaction, Ocr, were obtained from the spectral sequences on successive irradiation with the 458-, 488-, and 514-nm lines of an Ar ion laser and are shown in Table VII. Within this spectral range they lie between 0.004 and 0.006 with a tendential increase toward the shorter wavelengths. Their low values are probably a consequence of the large number of nonreactive deactivation pathways for the cluster, but they may also reflect a highly efficient thermal back-reaction that regenerates the starting material during the period of photolysis. If the photolysis is performed with UV light (337 nm, N, laser) the situation changes completely since metal to Os-CO (a*)
2101
2065
2029
1993
1957
Wavenumber, cm-' Figure 5. (a, top) UV-visible changes accompanying the 458-nm photolysis of 2.15 X M [PPN],[OS,,H~,C,(CO)~~] (2) in deoxygenated CH2CI2at 293 K. Spectra were recorded at 30-s intervals. (b, bottom) Infrared spectral changes accompanying the 5 14-nm photolysis of 2, in deoxygenated CH2C12at 20 OC. Spectra were recorded at 120-s inter-
vals. transitions and consequent dissociation of carbonyl ligands compete with the demercuration reaction. This manifests itself in an increased degree of irregularity in the spectral sequence and consequent loss of sharp isosbestics, a problem that cannot be fully avoided by performing the experiment in a solution that was saturated with CO prior to the irradiation. In the presence of an excess of P(OMe), (0.1 M in CH,CI,) as a scavenging ligand the cluster degradation due to CO loss is suppressed, and the photolysis appears to progress as in the experiments with visible = 2.8 X which is light, albeit with a quantum yield of aCr approximately one order of magnitude smaller. The O,, obtained is to be interpreted as an effective value for what is probably a mixture of 2 and various in situ generated phosphite substituted derivatives. In order to be competitive with the radiationless decay of the Os-CO u* states to the lower lying metal centred states responsible for the Hg ejection, the Os-CO breaking reaction must take place rapidly. Indeed, the low quantum efficiency on UV excitation supports the notion that the upper excited state levels undergo efficient CO ligand extrusion, thereby reducing the extent of photochemical conversion to the metal centered levels which effect the Hg ejection; in other words, on UV photolysis most of the chemical reactions that occur are ligand substitutions. In order to ensure that the scavenger did not interfere with the demercuration process, the photolyses at 458, 488, and 5 I4 nm were repeated in the presence of 0.1 M P(OMe)3. The quantum yields are identical within experimental error to those obtained earlier, which implies that the observed reaction is not affected by the phosphite. The Photolysis of [PPNl[Os18Hg,Cz(C0)42] (6). On oxidation of 2 with Ag(0,CCF3) the dominant absorption band in the UV-vis spectrum is shifted from 540 to 526 nm, accompanied by a slight decrease in the extinction coefficient for this band from 4.38 X IO4 to 4.04 X IO4 L mol-l cm-I. The irradiation of solutions
J . Am. Chem. Soc. 1991, 113, 8704-8708
8704 1.0
-----
L
7 unstable in the presence of P(OMe)3 and therefore analogous
'
scavenging experiments were not feasible, this quantum yield value can only be regarded as a rough estimate of the photochemical demercuration process. Conclusions
0
. i -
400
500
600
700
800
Wavelength, nm
Figure 6. UV-visible changes accompanying the 458-nm photolysis of 2.35 X M [ P P N ] [ O S , ~ H ~ , C ~ ( C(6) O )in~ ~deoxygenated ] CH2CI2 at 293 K. Spectra were recorded at 30-s intervals.
of the in situ generated monoanion 6 to generate 7 led to the complete disappearance of this band and the emergence of a weaker absorption band with its maximum at 570 nm. In the later stages of the photolytic conversion to the product the reaction occurred less cleanly than for the dianion 2. The photolysis sequences, as e.g. shown in Figure 6, were therefore not carried through to the complete conversion of the starting material in the quantum yield determinations. The quantum yields for the three Ar ion laser lines lie between a,, = 3.6 X and 4.6 X (Table VII), i.e. slightly below those determined for 2. Irradiation at 337 nm again led to a competition between metal-ligand dissociation processes and the demercuration, with an observed +cr value of 2.0 X Since the oxidized cluster proved to be
The aim of this study was the synthesis of systems and the development of methods which allow selective metal core transformations in large mixed-metal clusters. The cluster dianion [ O S ~ ~ H ~ ~ C ~ (has C Obeen ) ~ ~shown ] ~ - to undergo photolytic demercuration to the corresponding dimercury cluster, a reaction that is thermally fully reversible and therefore provides the first example of photochromism in the chemistry of high-nuclearity clusters. The fact that this type of chemistry is feasible for two different oxidation states of the parent compound motivates further investigations into the redox and photochemistry of this and related systems. Such work is currently underway. Acknowledgment. L.H.G. gratefully acknowledges the award of a Ph.D. scholarship by the Studienstiftung des deutschen Volkes and financial support by IC1 PIC. M.McP. thanks the Science and Engineering Research Council (U.K.) for financial support, and A.J.L. thanks the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U S . Department of Energy (Grant DEFG02-89ER14039), for support of this work. We are also grateful to Prof. M. E. Starzak (SUNY) for the use of the N, laser.
Supplementary Material Available: Tables of atomic coordinates and isotropic thermal parameters, anisotropic thermal parameters, and selected interbond angles (8 pages); listing of observed and calculated structure factors (25 pages). Ordering information is given on any current masthead page.
Synthesis and Characterization of the Mononuclear Compounds t-Bu2MOAr ( M = A1 or Ga, Ar = Bulky Aryl Group): Are the Short A1-0 and Ga-0 Bonds a Consequence of dnteractions? Mark A. Petrie, Marilyn M. Olmstead, and Philip P. Power* Contribution from the Department of Chemistry, University of California, Davis, California 95616. Received May 17, 1991 Abstract: The synthesis and characterization of the compounds t-Bu,MOAr (Ar = 2,6-t-BuZ-4-XC6HZ,M = AI, and X = t - B u , 1; Me, 2; H, 3; X = Me and M = Ga, 4) are described. They were characterized by X-ray crystallography (1, 2, and 4) and 'H NMR and IR spectroscopy. In the crystal, the structures of 1, 2, and 4 are monomeric with essentially trigonal
planar coordinations at the metals and short AI-0 and Ga-0 bond lengths that are near 1.71 and 1.82 A. The structural data together with VT 'H N M R studies indicate that there is probably little x-character in the M - 0 bonds. The shortness may be explained on the basis of an ionic bonding contribution and the low coordination number of the metal, with M-0 n-interactions playing a relatively minor role. Crystal data with Mo K a ( A = 0.71069 A) at 130 K: 1, C26H47AI0,a = 11.386 (lo), b = 29.1 1 (2), c = 24.969 (16) A, p = 102.05 (6)', monoclinic, space group Pn,Z = 12 (six molecules per asymmetric unit), R = 0.105; 2, C23H41A10, a = 8.200 (2). b = 17.876 (4), c = 15.910 (4)A, @ = 94.47 ( 2 ) O , monoclinic, space group PZ,/c,Z = 4, R = 0.047; 4, C2,H4,Ga0,a = 8.215 ( I ) , b = i7.738 (6), c = 15.921 (4) A, p = 94.73 (2)", monoclinic, space group PZl/c, Z = 4, R = 0.051. Introduction
The synthesis and structural characterization of compounds that have multiple bonding involving the heavier main group elements has been one of the major developments in inorganic/ organometallic chemistry in recent years.' Much of this effort ( I ) For example, (a) Goldberg, D. E.; Harris, D. H.; Lappert, M . F.; Thomas, K. M . J . Chem. SOL-.,Chem. Commun. 1976, 261 (Sn-Sn double bonds). (b) West, R.; Fink, M. J.; Michl, T. Science (Wushingron, D.C.)1981, 1343 (Si-Si double bonds). (c) Yoshifuji, M.; Shima, 1.; Inamoto, N.; Hirotsu, K.; Higuchi, T. J. Am. Chem. SOC.1981, 103, 4587 (P-P double bonds).
0002-7863/91/1513-8704$02.50/0
has concerned members of the phosphorus2or silicon groups3 The elements of the aluminum group have received much less attention in spite of the fact that their lighter congener boron is known to form multiple bonds to heavier main group elements such as sulfur," p h o s p h ~ r u sand , ~ arsenic6 as illustrated. Mes \ Mes,B=SLph
[
Mes \ Mes/B=E\Ph
E = P or AS
0 1991 American Chemical Society
]
